Then in 2006 something significant happened, in theory: the Supreme Court ordered Argentina's authorities to clean up Riachuelo once and for all.

It isn't an easy goal.

Water in the 2,300-square-kilometer (888-square-mile) basin is at least 50 percent above permitted levels of mercury, arsenic, zinc and lead, according to environmental pressure group Greenpeace.

The toxic cocktail that five million people are live in, drink and breathe, includes not just unregulated housing and raw sewage but some 20,000 industries, according to the government.

Locals say it may be the death of them, if a latest move toward clean-up doesn't succeed.

"Going down to the riverbed in Riachuelo is like sinking into thick black yogurt," said Guillermo Balbi, a professional diver who has worked in the area.

There are also oil refineries and chemical storage plants soiling the river basin; one makeshift settlement is grimly named Flammable Slum—"Villa Inflamable". For hundreds, this is home.

Hope, promises, & bad memories

Now, it will be up to a new government panel, which has emerged on the heels of the high court order, to see if real strides can finally be made in the name of so many people living in this environment.